High-density information discs are now commercially available with recorded video, audio and color information which can be played back through a standard television monitor. The density of the information recorded is such that the record can play up to one hour or even longer on each side while spinning at very high speeds of 450 rpm or higher. It will be appreciated that the size of the information bits recorded in the surface is very small, orders of magnitude smaller than are required for audio discs for example, which play back for about 15 minutes at 33 rpm.
In the manufacture of audio records, the audio information is first cut into a lacquer surface. This surface is then replicated by depositing a conductive layer, e.g., silver, on the lacquer and plating nickel over it, thus forming a self-sustaining negative part known as a master. One or more masters can be pulled from a single recorded substrate. The master can itself be replicated in turn to form a positive part known as the mother and finally, by a similar process, a negative metal stamper is plated onto the mother. This stamper is used in a molding press to mold plastic discs having the information pressed into the surface. This general scheme is also followed in the manufacture of high-density information discs, except that because of the size and density of the information bits, the original cutting is done in a finely grained electro-deposited bright acid copper substrate.
U.S. Pat. No. 4,305,791, herein incorporated by reference, describes a cleaning and passivating procedure for preparation of the copper substrate for subsequent nickel plating to form the master. The surface of the copper must be cleaned of all debris and grease prior to replication, and its surface must also be passivated to ensure that the nickel master can be completely parted from the copper with a minimum of damage to the master or the recording substrate. The preferred passivating procedure described includes a degreasing step using organic solvents, followed by a passivation step whereby the copper substrate is immersed in an alkaline cleaner, such as trisodium phosphate, and an oxidizing agent, such as potassium dichromate.
In an effort to improve the number of masters that can be plated onto a single copper recording substrate, it has been suggested by Gorog and Fox in co-pending application Ser. No. 348,677 filed Feb. 16, 1982, now U.S. Pat. No. 4,402,798 also herein incorporated by reference, that a very thin layer of chromium or chromium oxide be applied to the surface to harden and protect the copper. A layer about 200-400 angstroms thick can be deposited on the copper by vacuum evaporation, chemical vapor deposition, or most preferably, by sputtering.
However, the chromium-containing layer does not adhere well to the copper surface which has been passivated by the above-described method, and thus the substrate and master may be damaged when the parts are separated.